Power sources typically convert a power input to a necessary or desirable power output tailored for a specific application. In welding applications, power sources typically receive a high voltage, alternating current, (VAC) signal and provide a high current welding output signal. Around the world, utility power sources (sinusoidal line voltages) may be 200/208 V, 230/240 V, 380/415 V, 460/480 V, 500 V and 575 V. These sources may be either single-phase or three-phase and either 50 or 60 Hz. Welding power sources receive such inputs and produce an approximately 10-75 volt, DC or AC high current welding output.
There are many types of welding power sources that provide power suitable for welding. Some prior art welding sources are resonant converter power sources that deliver a sinusoidal output. Other welding power sources provide a squarewave output. Yet another type of welding power source is an inverter-type power source.
Converter-type power sources are particularly well suited for welding applications. Converter-type power sources can provide an ac square wave or a dc output. Converter power sources also provide a relatively high frequency stage, which provides a fast response in the welding output to changes in the control signals.
There are several well known types of converter power sources that are suitable for welding. These include boost power sources, buck power sources, and boost-buck power sources. Traditionally, welding power sources were designed for a specific power input. In other words, the power source cannot provide essentially the same output over the various input voltages. More recently, welding power sources have been designed to receive any voltage over a range of voltages, without requiring relinking of the power supply. One prior art welding power supply that can accept a range of input voltages is described in U.S. Pat. No. 5,601,741, issued Feb. 11, 1997 to Thommes, and owned by the assignee of the present invention, and is hereby incorporated by reference.
Many prior art welding power supplies include several stages to process the input power into welding power. Typical stages include an input circuit, a pre-regulator, a convertor and an output circuit that includes an inductor. The input circuit receives the line power, rectifies it, and transmits that power to the pre-regulator. The pre-regulator produces a dc bus suitable for conversion. The dc bus is provided to the convertor of one type or another, which provides the welding output. The output inductor helps provide a stable arc.
International safety requirements for welding machines often require the open-circuit/output voltage to not exceed 113 volts. However, welding power supplies used for stick welding typically should be able to provide at least a 65-75 volt output under load. When a forward converter is used the peak secondary voltage must be about twice the output voltage (i.e. 130-150 volts) because the pulse width may be 50% at most. This creates the potential for unacceptably high open circuit voltages. Prior art systems attempted to meet safety standards by providing a very narrow pulse width. However, even a narrow pulse could provide and unsatisfactorily high voltage when there is no load. Moreover, the narrow pulse may cause difficulty when trying to initiate a stick welding process.
The problem with high OCV is exacerbated because small capacitors are connected to the output terminals and chassis ground for suppression of high frequency interference from TIG arc starters. These capacitors will tend to be charged to the peak voltage applied to the secondary and hold this voltage.
Some prior art power supplies have a pre-load resistor that pre-loads the inverter, so that the capacitors don't charge to the peak. This resistor can present a sizable load, and a significant amount of wasted power, because of the relatively high voltage at the secondary. Also, the ripple voltage on the output may exceed the limits allowed, even with a pre-load resistor. Other prior art power supplies use an open circuit voltage regulator which electronically regulate the output to a lower limit using an error amplifier and voltage feedback. The error amplifier reduce the pulse width of the inverter and/or forces pulses to be skipped to bring the output voltage down. However, this makes the error amplifier sluggish because of the high gain of the power circuit at no load. Additionally the average output voltage may have to be reduced to a lower level to keep the peak ripple within limits. The lower level average output and/or sluggish error amplifier tend to cause difficulty with arc initiation, because the main inverter take a significant amount of time to "come alive" and to provide arc current.
Prior art pre-regulator stages typically include switches used to control the power. The losses in switches can be significant in a welding power supply, particularly when they are hard switched. The power loss in a switch at any time is the voltage across the switch multiplied by the current through the switch. Hard switching turn-on losses occur when a switch turns on, with a resulting increase in current through the switch, and it takes a finite time for the voltage across the switch to drop to zero. Soft switching attempts to avoid turn-on losses by providing an auxiliary or snubber circuit with an inductor in series with the switch that limits the current until the transition to on has been completed, and the voltage across the switch is zero. This is referred to as zero-current transition (ZCT) switching.
Similarly, hard switching turn-off losses also occur when a switch turns off, with a resultant rise in voltage across the switch, and it takes a finite time for the current through the switch to drop to zero. Soft switching attempts to avoid turn-off losses by providing an auxiliary or snubber circuit with a capacitor across the switch that limits the voltage across the switch until the transition to off has been completed, and the current through the switch is zero. This is referred to as zero-voltage transition (ZVT) switching. Soft switching has not been adequately employed in welding power supplies because the circuits require expensive additional switches and/or have losses in the circuitry that help create the zero current/voltage transitions.
When it is not practical or cost effective to use a true ZCT and ZCT circuit, an approximation may be used. For example, slow voltage/current transitions (SVT and SCT) as used herein, describe transitions where the voltage or current rise is slowed (rather than held to zero), while the switch turns off or on.
A typical prior art welding power supply 100 with a pre-regulator 104 is shown in FIG. 1. An input line voltage 101 is provided to a rectifier 102 (typically comprised of a diode bridge and at least one capacitor). Pre-regulator 104 is a hard-switched boost converter in the preferred embodiment, which includes a switch 106 and an inductor 107. A diode 108 allows a capacitor 109 to charge up by current flowing in inductor 107 when the switch 106 is turned off. The current waveform in inductor 107 is a rectified sinusoid with high frequency modulation (ripple).
The amount of ripple may be reduced by increasing the frequency at which switch 106 is switched. However, as the frequency of which a prior art hard switched boost converter is switched is increased to reduce ripple, the switching losses can become intolerable.
Another drawback of prior art power supplies is a poor power factor. Inductor 107 is part of the load seen by line voltage source 102, and thus the load is inductive in nature. Generally, a greater power factor allows a greater power output for a given power input. Also, it is generally necessary to have more power output to weld with stick electrodes having greater diameter. Thus, a higher power factor will allow a given welding power supply to be used with greater diameter sticks, and obtain the same performance obtained using smaller diameter sticks.